Fibrous nonwoven webs are well known for a wide variety of end uses, such as wipes, surgical gowns, dressings, etc. Fibrous nonwoven webs have been formed by a variety of processes including meltblowing and spunbonding.
In the spunbonding process a multiplicity of continuous thermoplastic polymer strands are extruded through a die in a downward direction onto a moving surface where the extruded strands are collected in a randomly distributed fashion. These randomly distributed strands are bonded together by thermobonding or by needlepunching to provide sufficient integrity in a resulting nonwoven web of continuous fibers. One method of producing spunbonded nonwoven webs is disclosed in U.S. Pat. No. 4,340,563. Spunbonded webs are characterized by a relatively high strength/weight ratio, isotropic strength, high porosity, good abrasion resistance and are useful in a wide variety of applications including diaper liners, street repair fabric and the like.
The meltblowing process differs from the spunbonding process in that polymeric webs are produced by heating the polymer resin to form a melt, extruding the melt through a die orifice in a die head, directing a fluid stream, typically an air stream, toward the polymer melt exiting the die orifice to form filaments or fibers that are discontinuous and attenuated, and depositing the fibers onto a collection surface. Bonding of the web to achieve integrity and strength occurs as a separate downstream operation. Such a meltblown process is disclosed in U.S. Pat. No. 3,849,241. Meltblown webs are characterized by their softness, bulk absorbency, and relatively poor abrasion resistance and are useful for product applications such as surgical drapes and wipes.
U.S. Pat. No. 4,863,785 discloses a nonwoven composite material with a melt-blown fabric layer sandwiched between two prebonded, spunbonded reinforcing layers, all continuously-bonded together. The spunbonded material requires prebonding, and no parameters or methods of measurement of uniform basis weight are identified.
A major limitation that can be observed in many commercially available spunbonded webs is nonuniform coverage, such that areas of coverage in the fabric which are thicker or which are thinner are very noticeable, giving the webs a "cloudy" appearance. Basis weight of the spunbonded webs can vary significantly from one region of the web to another. In many applications, attempts are made to compensate for the poor fabric aesthetics and physical properties that result from this nonuniformity of coverage and basis weight by using webs having a greater number of filaments and a heavier basis weight than would normally be required by the particular application if the web had a more uniform coverage and basis weight. This, of course, adds to the cost of the product and contributes to stiffness and other undesirable features.
Meltblown fabrics, in constrast, are more uniform in coverage but have a limitation of low tensile strength. Many lower basis weight meltblown webs are marketed as composite fabrics with the low basis weight meltblown web sandwiched between two layers of spunbonded fabric to provide sufficient strength for processing and end use.
U.S. Pat. No. 4,790,736, incorporated herein by reference, discloses an apparatus for centrifugal fiber spinning of various thermoplastic resins with pressure extrusion for producing continuous nonwoven fabrics. Filament or fiber deniers ranging in value from 5 to 27 g/9000 m and a two-ply, lay-flat fabric having a basis weight of 0.75 oz/yd.sup.2 produced from nylon-6 polymer are disclosed. These nonwoven webs have good strength and coverage, particularly at basis weights above 1 oz/yd.sup.2 ; however, greater uniformity of coverage at lower basis weights would be desirable.
In view of the limitations of the spunbond and meltblown fabrics produced by known processes, there is a need for a self-bonded, fibrous nonwoven web material having very uniform basis weight properties and balanced physical properties, such that physical properties in the machine direction are approximately the same as properties in the cross machine direction, an improved process to prepare same and composite products comprising the nonwoven material bonded to at least one additional fabric, film or nonfabric material.
As used herein, a nonwoven web having uniform basis weight is taken to mean a nonwoven web which has a Basis Weight Uniformity Index (BWUl) of 1.0.+-.0.05, wherein the BWUl is defined as a ratio of an average unit area basis weight determined on a unit area sample of the web to an average area basis weight determined on an area sample, N times as large as the unit area sample, wherein N is about 12 to about 18, the unit area sample has an area of 1 in.sup.2, and wherein standard deviations of the average unit area basis weight and the average area basis weight are less than 10% and the number of samples is sufficient to obtain average basis weights at a 0.95 confidence interval. For example, for a nonwoven web in which 60 samples of 1 in.sup.2 squares determined to have an average basis weight of 0.993667 oz/yd.sup.2 and a standard deviation (SD) of 0.0671443 (SD of 6.76% of the average) and 60 samples of 16 in.sup.2 squares (N was 16) determined to have an average basis weight of 0.968667 oz/yd.sup.2 and a standard deviation of 0.0493849 (SD of 5.10% of average), the calculated BWUl was 1.026.
Accordingly, it is an object of the present invention to provide a self-bonded, fibrous nonwoven web having a very uniform basis weight and tensile properties which are more evenly balanced in the machine and cross machine directions.
Another object of the present invention is to provide a self-bonded, fibrous nonwoven web comprising a plurality of substantially continuous polymeric filaments having a uniform basis weight of 0.1 oz/yd.sup.2 or greater wherein the polymeric filaments comprise a thermoplastic selected from the group consisting of polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, polyamide, polyester, a blend of polypropylene and polybutene, and a blend of polypropylene and linear low density polyethylene.
A further object of the present invention is to provide a uniform basis weight self-bonded, fibrous nonwoven web for use in composite products in which the nonwoven web is bonded to at least one additional fabric, film or nonfabric material.
A still further object is to provide an improved method for producing a self-bonded, fibrous nonwoven web having a very uniform basis weight.